Pollution and Purification of Quenching Oil

In the current production of heat treatment enterprises, quenching oil, as a quenching medium, accounts for about 70% according to statistics, and it can be seen that the amount of quenching oil still accounts for a considerable proportion.  

Checking the characteristics of quenching oil regularly is the key to controlling the heat treatment process and ensuring the quality of quenching workpiece.  However, most of the enterprises in the whole heat treatment industry generally ignore the performance of the quenching medium, although they invest a lot of money to improve the equipment and quality control means, but rarely care about the state of the quenching medium.  Often a quenched oil is used for many years and is kept in its state only by adding new oil.  Many workpiece quality problems produced by heat treatment are almost related to the state of quenched medium and often occur repeatedly.  For high strength fasteners of grade 10.9 or above or heat treatment with special requirements, the performance of quenching medium plays an important role in the success or failure of heat treatment operation.  

The function of quenching oil is to quickly take away the heat of the red hot metal parts, so that it can be reduced to the martensite transformation temperature to obtain high hardness martensite structure and hardened layer depth, but also to reduce the deformation of parts and prevent cracking.  Therefore, the basic characteristic of quenching oil is "cooling characteristic", which is characterized by faster cooling rate in high temperature stage and slower cooling rate in low temperature stage. This characteristic is very suitable for the quenching requirements of alloy steel ≥10.9 grade high strength fasteners.  

Quenching oil changes in the use of more complex, thermal decomposition, oxidation and polymerization reaction, it contains the additives and base oil changes of two parts, is the comprehensive result of changes in these two aspects, different quenching oil and different quenching process, the deformation will have bigger difference, thus strengthen the strict management of quenching oil is very important to guarantee the quality of the quenching workpiece.  

Daily quenching oil quality monitoring items are viscosity, flash point, moisture, insoluble ash (also known as sediment) and cooling rate.  

1.  Flash point  

Under normal circumstances, regardless of open or closed quenching oil grains, with the extension of service time, the flash point of oil tends to decline.  The flash point of oil grains in continuous mesh belt furnace can be stabilized after dropping 10~20℃.  When the flash point drop is more prone to quenching soft point, and unsafe, should be treated in time.  

2.  viscosity  

This index is closely related to the cooling performance of oil, and the change of viscosity is different for open and closed oil tanks.  The viscosity of open tank tends to increase because of the fast burning loss and the high probability of oxidation polymerization.  The quenching oil of closed oil tank is mainly thermal decomposition, so the viscosity shows a downward trend. Thermal decomposition reduces the viscosity, resulting in the generation of soft point and the increase of deformation.  Additive decomposition viscosity increases, resulting in reduced cooling capacity of the oil.  The higher the temperature at which oil is used, the faster it deteriorates.  Continuous mesh belt furnace is equipped with a complete loop cooling system, so that the oil temperature is stable in the range of 60~70℃, and through the loop stirring tank to prevent local overheating and other measures, can slow down the deterioration of oil, prolong the service life of oil.  

3.  Insoluble ash (precipitate)  

The insoluble ash (precipitate) in the oil includes coke mud, fine iron dust particles and dust particles in the tank.  When these sundries exceed a certain number, they will cause the viscosity of oil to rise and the cooling capacity to decline. Therefore, it should be removed regularly by filtering oil and cleaning the oil tank.  

4.  The moisture  

Trace water in the oil will seriously affect the cooling performance of the oil, resulting in reduced brightness of the workpiece after quenching, uneven hardness, soft spot and even cracking tendency.  It is found that the deformation problem caused by oil quenching is partly caused by water content in oil.  In addition, water in oil accelerates the emulsification and deterioration of oil and promotes the failure of additives in oil.  When the water content in the oil is greater than or equal to 0.1%, when the oil is heated, the water gathered at the bottom of the oil tank may reach the boiling point, and the volume suddenly expands, which is easy to cause oil overflow quenching oil tank and cause fire.  

Quenching oil contains water, which mainly deteriorates the cooling performance and accelerates the aging of oil.  For quick effect on the cooling characteristics of quenching oil water cut the results of the study showed that the failure trace water prompted additive, lead to the highest cold quenching oil speed decreases significantly, with the increase of water content in oil, increase the cooling rate of convection cooling stage (300 ℃ cooling rate from 8 ℃ / s increased when the water to water cut 0.20% of 40 ℃ / s),  The workpiece deformation and cracking tendency will also increase.  

If the quench oil is mixed with more than 200 PPM of water, it is necessary to find out the cause of the water. If the water content in the oil reaches 0.5%, it is necessary to try to remove the water in the oil.  Usually heat dehydration, precipitation dehydration, centrifugal dehydration, filtration dehydration, which will be beneficial to ensure hardening, reduce deformation and cracking.  

5.  The cooling curve  

The cooling curve of rapid quenching oil can directly indicate the cooling characteristics of the quenching oil, which is an important means to select the quenching oil and monitor the cooling deformation of the oil.  Therefore, the cooling characteristic curve of quenching oil should be tested regularly by heat treatment enterprises.  

Quenching oil in use due to thermal decomposition, oxidation and polymerization reaction, oxide skin, carbon residue dissolved in the oil, so that the color of the oil black, resulting in cooling characteristics decline;  At this time, the strength, hardness, toughness, wear resistance of the workpiece after quenching can not reach the specified index, and even cause deformation and cracking of the workpiece.  The common practice of many enterprises is to add new oil to the quenching pool (tank), alleviate the decline rate of oil performance, this kind of palliation, often is the quality of the workpiece has not reached the specified quality indicators;  The quenching oil must be extracted, the oil pool (tank) cleaned, and the new oil can be added to ensure the quality of the workpiece.  

Replace the waste oil can be used after purification by the decolorization clean oil machine. The decolorization clean oil machine is mainly used for the pollutants of oil products:  Such as water, oxide scale, carbon residue, gum, asphaltene removal, after purification, oil color recovery, close to the new oil color, the index reaches the national standard (SH 0564 heat treatment oil), can be used as new oil, ensure the quenching oil index and workpiece quality, do not need to invest a lot of money to buy new oil to replace.  

Decolorizing oil purifier adopts advanced dual primary dehydration technology to completely remove the water in quenching oil and restore the cooling performance of quenching oil;  The flocculation and adsorption technology is adopted to remove the oxide scale, carbon residue, colloid and other impurities dissolved in oil, restore the flash point, viscosity and cooling curve of quenched oil, and ultimately ensure the recovery of quenched oil index and physical and chemical properties, and ensure the quality of the workpiece.  

conclusion  

The cooling characteristic of quenching oil is very important to the success of heat treatment of workpiece.  Heat treatment enterprises (or heat treatment workshops) should establish a complete set of quenching oil management system and records, improve and maintain the quenching oil tank has a good mixing, heating and cooling system, to ensure that the quenching oil tank is in the process control state.  

In addition, according to the amount of heat treatment output, every six months or a year to clean up the oil tank, filter oil, remove the bottom of the tank residue debris, timely supplement new oil.  Every month or every three months for each oil tank should be taken oil sample detection once to monitor the change of oil quality, when the detection index exceeds the specified standard and can not be adjusted and improved, the whole tank must be changed oil, with decolorizing oil purification machine after the waste oil in filling.  This not only ensures the index and quality of quenching oil, but also saves a large amount of money for enterprises to purchase quenching oil. It is a multi-basis quality control and energy saving measure.